Add v1.0 to v2.0 migration guide

[chrisyamas]

Pull Request

Summary

Adds a v1.0 to v2.0 migration guide to the documentation site, under Specification > Migrating to v2.0.

The guide covers every change staged for v2.0:

• passenger_events to device_events rename and device-centered model (Rename passenger_events to device_events with expanded scope #251)
• event_type snake_case enum mapping table (Update event_type enum values to snake_case #247)
• stop_visits duration field renames (Rename duration fields with explicit suffix #249)
• vehicles / vehicle_groups / vehicle_assignments / consist_vehicles restructure (Update handling of vehicles and consists #272)
• new optional capabilities: vehicle_crew (Add vehicle_crew supporting table #246), device_status (Add device_status table for tracking device availability #253), vehicle_label (Add new vehicle_label field to vehicles table #250)
• non-normative event_count clarifications (Clarify event_count usage and APC data guidance #248)

It includes migration checklists for both producers and consumers.

Notes for reviewers

• Opened as a draft since the constituent v2.0 PRs are still in review; field names in the guide track the current state of each PR and the guide should be finalized only once those merge. Do not merge before the PRs it describes.
• Non-normative change (docs only, plus one mkdocs nav entry).

Review checklist

• Guide accurately reflects each merged v2.0 change at release time
• Reviewed and approved by 2+ contributors or board members

[github-actions]

Documentation available at: http://tides-transit.github.io/TIDES/feature/v2-migration-guide